Color TFT liquid crystal display devices and the like are produced by laminating a color filter substrate and a TFT array substrate. TFT array substrates are generally provided with an interlayer insulating film for insulation between wirings disposed in layers. As a material for forming the interlayer insulating film, a positive-type radiation-sensitive composition has been widely used since the number of steps for obtaining a required pattern configuration can be diminished, and additionally an interlayer insulating film having sufficient flatness is preferred.
As a component of such a radiation sensitive composition for forming an interlayer insulating film, acrylic resins have been primarily used (see, JP-A No. 2001-354822 (the term “JP-A” as used herein means an “unexamined published Japanese Patent Application”)). To the contrary, attempts have been made to use as a component of the radiation sensitive composition a polysiloxane-based material that is superior in heat resistance and transparency than acrylic resins (see, JP-A No. 2006-178436, JP-A No. 2006-276598, and JP-A No. 2006-293337). Siloxane polymers used in conventional polysiloxane-based radiation sensitive compositions are obtained by subjecting a silane compound such as phenyltrimethoxysilane to hydrolytic condensation. In order to keep the surface hardness of the cured film obtained from a radiation sensitive composition containing such a siloxane polymer at a level appropriate in practical applications, it is important to control the amount of a silane compound having phenyl groups such as phenyltrimethoxysilane used in the synthesis of the siloxane polymer. It is reported that when the content of phenyl groups relative to Si atoms in a siloxane polymer is greater than 60% by mole, the surface hardness of the cured film is lowered (for example, see JP-A No. 2006-178436).
However, when the content of the phenyl groups relative to Si atoms in the siloxane polymer is adjusted to be from 5% by mole to no greater than 60% by mole only for the purpose of improving the surface hardness of the cured film, the following problems may occur. For example, a decrease in the content of phenyl groups results in lowering of the refractive index of the cured film, thereby contributing to lowering of the luminance of liquid crystal panels. Furthermore, in the case of positive-type radiation-sensitive compositions, since a siloxane polymer having a low content of phenyl groups is poorly compatible with a quinone diazide compound added as a photosensitizing agent, whitening of the cured film (opacification of the cured film that occurs by phase separation of the siloxane polymer and a quinone diazide compound) may be caused.
When the content of phenyl groups relative to Si atoms in the siloxane polymer is from 5% by mole to no greater than 60% by mole, in addition to the aforementioned problems, deterioration of durability of the cured film against a resist remover solution used in an ITO etching step becomes a great problem. In producing a TFT array substrate, following formation of an interlayer insulating film, an ITO (indium tin oxide) film is formed thereon to cover the entire face of the substrate. Thereafter, in order to remove unnecessary portions of the ITO film at, an etching liquid such as strong acid is used to carry out etching of the ITO film. Upon etching of the ITO film, necessary portions of the ITO film is protected using a novolak-based positive resist or the like so as not to etch necessary portions of the ITO film. Thereafter, the positive resist is removed by a resist remover solution. In this step, when the interlayer insulating film was formed from a siloxane polymer having a low content of phenyl groups, swelling and/or erosion of the cured film is caused by the resist remover solution, and thus alteration of the film thickness occurs between before and after the ITO etching step. The alteration of the film thickness of the cured film leads to a defective cell gap of the liquid crystal display device, which in turn causes nonuniformity of the display in the liquid crystal display device.
Additionally, in JP-A No. 2003-288991, for example, a radiation sensitive composition for forming an insulating film of organic EL display device, which contains a certain silane compound, and a compound that generates an acid or a base upon irradiation with a radiation is disclosed. However, no disclosure is found indicating that by controlling the amount of the silane compound having phenyl groups used, various performances such as heat resistance, transparency and low dielectricity generally demanded for interlayer insulating film can be improved, and in addition, refractive index, and resist remover solution resistance as well as dry etching resistance in ITO film etching steps can be particularly improved.
Under such circumstances, the development of a polysiloxane-based positive-type radiation-sensitive composition has been strongly desired, which is capable of forming a cured film having high heat resistance, transparency, and low dielectricity that are generally demanded for interlayer insulating films, as well as superior refractive index and resist remover solution resistance in ITO film etching steps, and also capable of forming a liquid crystal cell having a high voltage holding ratio, and also having high radiation sensitivity and high compatibility with quinone diazide compounds.